Exposure of cells to stresses such as heat shock leads to the induction of a group of proteins called heat shock proteins (Hsp). Hsp plays a role in restoring the denatured proteins caused by various stresses such as heat shock stress, oxidative stress, etc. to their normal state in cells.
Hsp90 (heat shock protein 90), which is one of heat shock proteins, binds to various substrate proteins (client proteins) in vivo and regulates their functions. Hsp90 client proteins are known to include glucocorticoid receptor, Akt, cycline dependent kinase 4, etc. (cf., e.g., Non-Patent Document 1).
Hsp90 forms complexes with heat shock factor-1 (HSF-1) and maintains HSF-1 in an inactive state as the monomer to downregulate a heat shock reaction (cf., e.g., Non-Patent Document 2).
A substance that binds to Hsp90 to inhibit the activity (Hsp90 inhibitor) can induce the expression of heat shock proteins including Hsp70, and is thus expected to be used as a cell protecting agent which exhibits a cell protecting activity (cf., e.g., Patent Document 1 and Non-Patent Documents 1 and 3). The Hsp90 inhibitor can mediate the activation of HSF-1 to induce the expression of a heat shock protein such as Hsp70 having a cell protecting activity.
In various diseases such as heart diseases, neurological disorders, inflammatory disorders, etc., cell or tissue injury caused by oxidative stress or the like has become a problem. Cell protecting agents are considered to be promising drugs for the prevention/treatment of these diseases. Heat shock proteins including Hsp70 inhibit apoptosis induced by various factors such as ultraviolet exposure, active oxygen, etc. to exhibit therapeutic effects in disease model animals with heart diseases, neurodegenerative disorders, diabetes, etc. Accordingly, the Hsp90 inhibitor is a heat shock protein inducing compound and could be a preventive/therapeutic drug for the diseases described above. However, Hsp90 inhibitors such as geldanamycin cause the degradation of Hsp90 client proteins to generate cytotoxicity, and are unsatisfactory as cell protecting agents.
As a screening method to search for a Hsp90 inhibitor available as a cell protecting agent, there is a method for assaying the binding property of a test compound to Hsp90 (hereinafter “Hsp90 binding activity”). However, it cannot be determined merely by assaying the Hsp90 binding activity if a test compound would cause the degradation of Hsp90 client proteins to show cytotoxicity.
In view of the foregoing circumstances, a screening method has been sought to find an Hsp90 inhibitor having a minimal activity of promoting the degradation of an Hsp90 client protein and a minimal cytotoxicity and thus useful as a cell protecting agent.    Non-Patent Document 1: Heat shock proteins as emerging therapeutic targets, Br. J. Pharmacol., 2005; 146: p. 769-780    Non-Patent Document 2: Regulation of cellular functions by molecular chaperones, 132, 2001    Non-Patent Document 3: Inhibitors of the Hsp90 molecular chaperon: current status, Adv Cancer Res., 2006; 95:p. 323-348    Patent Document 1: International Publication No. WO 2004/069999